Transformer questions

[juan123]

ok 2 simple questions:

1. Why do some transformers such as those universal ones cause "humming" in say a pocket radio when they are the power supply?

2. My clock radio's transformer bit the dust. I'm wondering, how hard is it to rewind the transformer? Is it something that pretty much a newbie could do?

Thats all for today... Thanks!

 

[Externet]

1- Because the DC is not smoothed enough by the filtering circuit, or it is underrated.

2- The transformer can be replaced by transplanting one of those inside a 'universal' power supplies but the cause of burning must be repaired first; or the bad transformer can be eliminated and feed the radio by adding a power jack and an external universal supply.

 

[Sceadwian]

You could rewind the transformer if you wanted but it'd probably be easier to just to yank a working one out of a similar device, they generally speaking are 12 volt transformers though you need to determine the voltage before you do anything. Difficulty is pretty much across the board depending on what if any potting compound was used for the windings (some of them are downright messy) To properly copy the old transformer you have to determine the wire gauge of the primary and secondary, and the number of turns in each. What you're really trying to do is match the DC resistance, total number of turns and turns ratio as closely as possible. If your transformer is toast though that might be a problem as you're not going to be able to determine all of those. I'd match it with a transformer that's roughly the same size rated at 12 volts and see what happens.

 

[Sceadwian]

The hum from 'cheap' power supplies can generally be reduced or eliminated by using large filter capacitors on the output leads. If you have the space large chokes feeding a large capacitor can really smooth things out.